One of the three branched-chain amino acids, notable for its effect on glucose uptake into muscle. Rarely supplemented alone, and there is little reason to.
Typical dose
1.0–5.0 g per day
With protein feedings, if used
What it is
L-isoleucine is an essential amino acid and one of the three branched-chain amino acids, alongside leucine and valine. It is found in all complete protein sources: meat, fish, eggs, dairy, soy and legumes.
In BCAA products it usually sits at the 1 position in the standard 2:1:1 ratio, meaning a 5 g serving supplies around 1.25 g.
What distinguishes isoleucine from its two companions is its effect on glucose. It promotes glucose uptake into muscle by a route largely independent of insulin, which is more interesting metabolically than anything specific to muscle building.
How it works
Isoleucine increases glucose uptake in skeletal muscle by promoting translocation of GLUT4 transporters to the cell membrane, and it appears to do this without requiring insulin, which is what makes it notable.
Like the other branched-chain amino acids, it is metabolised primarily in skeletal muscle rather than the liver, and can be used as a fuel substrate during prolonged exercise.
It contributes to muscle protein synthesis as one of the nine essential amino acids, but it does not trigger mTOR the way leucine does. Its value in a supplement context is as part of the complete essential amino acid pool rather than as a signalling molecule.
What the evidence shows
The glucose uptake finding is reasonably well demonstrated in animal and cell research, with some supporting human data. It is genuine, though its practical significance at supplemental doses in healthy people is unclear.
There is essentially no evidence supporting isolated isoleucine supplementation for muscle growth, strength or performance. Trials examining it separately from the other BCAAs are scarce.
Observational work has linked elevated circulating branched-chain amino acids to poorer metabolic markers, though whether that reflects cause or consequence remains contested. Neither that nor the glucose uptake finding should be over-interpreted.
Dosing
Isolated isoleucine doses of 1-5 g appear in research, but it is rarely sold alone and there is little reason to take it that way.
Within a 2:1:1 BCAA product, a 5-10 g serving supplies 1.25-2.5 g of isoleucine. Within a complete EAA product it appears in proportion to the other essentials.
The practical position is straightforward: adequate dietary protein supplies isoleucine comfortably, and if you want an amino acid supplement, a complete EAA product is a better use of the money than any single branched-chain amino acid.
At a glance
Common forms
Within BCAA blends (2:1:1), Within EAA blends, L-isoleucine powder, Capsules
Half-life / duration
Plasma peak within 30-60 minutes
Timing
With protein feedings, if used
Competition status
Permitted — not on the WADA Prohibited List
Category
Muscle & Recovery
Safety and side effects
Isoleucine is an ordinary dietary component and well tolerated at supplemental doses. Digestive discomfort is the occasional complaint at higher intakes.
As with the other branched-chain amino acids, large isolated doses compete with tryptophan and tyrosine for transport into the brain, which is one reason balanced EAA products are preferable to single BCAAs.
Because it promotes glucose uptake, take care if your blood sugar is being managed. Anyone advised to restrict specific amino acids should avoid it.
Who should avoid it
Avoid if you have been advised to restrict specific amino acids in your diet. Take care if your blood sugar is being managed. Speak to your doctor before use if you take regular medication or are under medical care. Not established for use in pregnancy or breastfeeding at supplemental doses.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
